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Open Access Research

Trans-packaging of human immunodeficiency virus type 1 genome into Gag virus-like particles in Saccharomyces cerevisiae

Naoki Tomo13, Toshiyuki Goto2 and Yuko Morikawa1*

Author Affiliations

1 Kitasato Institute for Life Sciences and Graduate School for Infection Control, Kitasato University, Shirokane 5-9-1, Minato-ku, Tokyo, 108-8641, Japan

2 School of Health Science, Faculty of Medicine, Kyoto University, Kawahara-cho 53, Shogoin, Sakyo-ku, Kyoto, 606-8507, Japan

3 Present address: Life Technologies Japan Ltd, Mita Twin Bldg., Shibaura 4-2-8, Minato-ku, Tokyo, 108-0023, Japan

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Microbial Cell Factories 2013, 12:28  doi:10.1186/1475-2859-12-28

Published: 26 March 2013

Abstract

Background

Yeast is recognized as a generally safe microorganism and is utilized for the production of pharmaceutical products, including vaccines. We previously showed that expression of human immunodeficiency virus type 1 (HIV-1) Gag protein in Saccharomyces cerevisiae spheroplasts released Gag virus-like particles (VLPs) extracellularly, suggesting that the production system could be used in vaccine development. In this study, we further establish HIV-1 genome packaging into Gag VLPs in a yeast cell system.

Results

The nearly full-length HIV-1 genome containing the entire 5 long terminal repeat, U3-R-U5, did not transcribe gag mRNA in yeast. Co-expression of HIV-1 Tat, a transcription activator, did not support the transcription. When the HIV-1 promoter U3 was replaced with the promoter for the yeast glyceraldehyde-3-phosphate dehydrogenase gene, gag mRNA transcription was restored, but no Gag protein expression was observed. Co-expression of HIV-1 Rev, a factor that facilitates nuclear export of gag mRNA, did not support the protein synthesis. Progressive deletions of R-U5 and its downstream stem-loop-rich region (SL) to the gag start ATG codon restored Gag protein expression, suggesting that a highly structured noncoding RNA generated from the R-U5-SL region had an inhibitory effect on gag mRNA translation. When a plasmid containing the HIV-1 genome with the R-U5-SL region was coexpressed with an expression plasmid for Gag protein, the HIV-1 genomic RNA was transcribed and incorporated into Gag VLPs formed by Gag protein assembly, indicative of the trans-packaging of HIV-1 genomic RNA into Gag VLPs in a yeast cell system. The concentration of HIV-1 genomic RNA in Gag VLPs released from yeast was approximately 500-fold higher than that in yeast cytoplasm. The deletion of R-U5 to the gag gene resulted in the failure of HIV-1 RNA packaging into Gag VLPs, indicating that the packaging signal of HIV-1 genomic RNA present in the R-U5 to gag region functions similarly in yeast cells.

Conclusions

Our data indicate that selective trans-packaging of HIV-1 genomic RNA into Gag VLPs occurs in a yeast cell system, analogous to a mammalian cell system, suggesting that yeast may provide an alternative packaging system for lentiviral RNA.

Keywords:
Yeast; HIV; Virus-like particle; Genome packaging